Over the past two decades, there has been an increasing interest in, and proliferation of, airborne laser altimeters, which measure the roundtrip time of flight of a laser pulse from an aircraft to a surface. High speed optical scanning devices of various types have been used to direct a train of high repetition rate (˜10 kHz) laser pulses to an array of points on the ground. Combined with GPS Differential positioning, these instruments routinely provide 3D topographic maps over wide areas with point spacings on the order of a few meters and vertical (range) accuracies on the order of a decimeter. The applications for these devices are numerous and include commercial surveying of populated areas, biomass estimation and forestry management through the measurement of tree canopies and subcanopies, changes in sea and lake levels, the determination of hydrological runoff routes with application to flood control and hazard estimation, and even personal security applications. Sophisticated software packages, tailored to specific applications, can, for example, strip the surface of manmade structures and vegetation, revealing hidden fault lines of interest to geologists and geophysicists. Airborne instruments can provide useful local, and even regional, data, but many in the Earth science community desire comparable data on a global scale.
A small number of laser altimeters have operated in space. Altimeter data from three manned Apollo missions and the 1994 Clementine mission has recently been processed to provide a crude topographical map of the Moon. In the early 1970's, three of NASA's manned Apollo missions (15, 16, and 17) carried a low repetition rate (16 to 32 second fire interval) ruby laser altimeter to the Moon and provided approximately 7080 measurements of the command module height relative to the lunar topography. In 1994, the Clementine Mission operated for about two months at a substantially higher, but still low, laser fire rate of 2 Hz and provided a more robust set of 70,000 altimetric measurements during approximately two months in lunar orbit. A similar altimeter on the Near Earth Asteroid Rendezvous (NEAR) mission has recently provided a detailed topographic map of the asteroid EROS.
The largest body mapped to date by a laser is the planet Mars. NASA's Mars Orbiting Laser Altimeter (MOLA) instrument on the Mars Global Surveyor (MGS) spacecraft recently completed the first global topographic map of a planet from a near-polar orbit using an actively Q-switched, diode-pumped Nd:YAG laser operating at a repetition rate of 10 Hz. With an orbital ground velocity on the order of 3 km/sec, the along-track spacing between consecutive measurements was about 300 m. Like other missions, the MOLA instrument did not include a cross-track scanner, and hence only terrain directly below the spacecraft was interrogated under normal operating conditions. Periodically, however, the entire spacecraft was tilted via ground command to obtain data at or near the Martian poles. As with any near-polar orbit, the spatial concentration of range measurements is large at higher planetary latitudes and falls off dramatically as one approaches the Martian equator. Nevertheless, MOLA provided the most accurate global vertical height datum ever achieved from planetary orbit and was an unqualified engineering and scientific success.
First generation altimetric approaches are not well-suited to generating the few meter level horizontal resolution and decimeter precision vertical (range) resolution on the global scale desired by many in the Earth and planetary science communities. The first generation spaceborne altimeters have been characterized by a laser operating in the infrared (1064 nm) at a few tens of Hz with moderate output energies (50 to 100 mJ), a telescope in the 50 to 100 cm range, and a single element (i.e. non-pixellated) detector which detects and processes multi-photon returns from the surface. On bare terrain, the signal waveforms reflect the slope and surface roughness within the laser footprint (typically several tens of meters in diameter) as well as any false slopes due to pointing error. On Earth, the presence of manmade buildings and volumetric scatterers (such as tree canopies or other vegetation) generally makes waveform interpretation more complex and difficult.
Clearly, one major challenge to the conventional approach is the sheer number of measurements required over a nominal mission lifetime of two to three years. For example, in order to generate a 5 m×5 m vertical grid map of Mars, which has a mean volumetric radius of 3390 km, over 7 trillion individual range measurements are required, even if one makes the unrealistic assumption that no ground spatial element is measured twice. In any realistic mission, the actual number of range measurements will be significantly larger since an instrument designed to provide contiguous coverage at the planetary equator will oversample the higher latitudes where the ground tracks are more narrowly spaced. If one were to simply scale conventional approaches, one would clearly face severe prime power, weight, and instrument longevity issues.
A second technical challenge is the high ground speed of the spacecraft (about 3 km/sec for a nominal 300 km altitude Mars orbit) coupled with the need to incorporate a scanner to cover the large area between adjacent ground tracks, especially near the equator. At a nominal altitude of 300 km, for example, the satellite would have an orbital period about Mars of approximately 113 minutes. Thus, a three year mission would produce 13,910 orbits or 27,820 equator crossings with an average spacing between ground tracks at the equator of 766 meters. The latter spacing corresponds to about 154 resolution elements (Δ=5 m) in the cross-track direction between adjacent ground tracks and further implies a minimum cross-track scan angle of about 0.15 degrees. For truly contiguous coverage using a conventional single element detector, these 154 cross-track measurements must be completed in the time it takes the spacecraft to move one resolution element in the along-track direction, or within 1.67 msec. This implies a rather daunting laser fire rate of 92.4 kHz. Furthermore, a uniformly rotating mechanical scanner, for example, must complete a half cycle of its movement within the same 1.67 msec period, i.e. an impossible 300 Hz (18,000 RPM) rate. While alternative non-mechanical scanners, such as electro-optic or acousto-optic devices, are capable of very high scanning speeds and have no moving parts, they fall far short of the angular range requirements, are highly limited in their useful aperture, and require fast high voltage or high RF power drivers.
An additional technical challenge stems from the high laser fire rate and the long pulse time of flight (TOF). At 300 km altitude, the laser pulse completes a roundtrip transit to the surface in 2 msec. Thus, for laser fire rates in excess of 500 Hz, multiple pulses will be in flight simultaneously. In principle, it is easy to associate the correct return pulse with the appropriate outgoing pulse provided the roundtrip satellite-to-surface TOF is known a priori to well within a single laser fire interval. For the 92.4 kHz rate derived previously, however, approximately 185 pulses would be simultaneously in transit, and it would be necessary to have a priori orbital knowledge at the 1.6 km level in order to unambiguously tie a given surface return to the appropriate output pulse. While such a navigation accuracy might be easy to achieve in Earth orbit using either Global Positioning System (GPS) receivers or Satellite Laser Ranging (SLR) to passive reflectors on the spacecraft, it would likely be a much more difficult challenge in orbits about extraterrestrial bodies.
A second technical problem associated with the longer pulse TOF from orbit is related to “transmitter point-ahead”, i.e. the offset between the center of the laser beam at the surface and where the receiver is looking one 2 msec round trip transit time later. For an unscanned system, the offset due to a 3 km/sec spacecraft ground velocity is only 6 m (slightly more than one resolution element) in the along-track direction and can be easily accommodated, either by a fixed offset of the transmitter in the positive along-track direction or by a modest increase in the receiver field of view (FOV). In the current example, however, the scanner must complete over half a cycle of its scan within the pulse TOF. Thus, the receiver FOV must be opened up to span the full 0.15 degree separation (766 m) between ground tracks in the cross-track dimension while the laser illuminates only a 5 m diameter circle within that FOV and defines the ground resolution element being interrogated. This approach greatly increases the solar background noise incident on the detector during local daytime operations relative to the unscanned case and elevates the laser output energy requirements for good discrimination of the signal. An alternative low noise approach would be to independently steer the transmitter and receiver, which will be discussed in later sections.